While some of the source data files prior to 1989 do have CMSA/MSA Rank codes, ranked by population (i.e. 1 = New York-N. New Jersey-Long Island, NY-NJ CMSA, 2 = Los Angeles-Anaheim-Riverside, CA CMSA, 3 = Chicago-Gary-Lake County, IL-IN-Wl), IPUMS-CPS does not currently offer this variable. However, it is possible to download the source data files from NBER and merge this information onto your IPUMS-CPS extract. Because the NBER and IPUMS-CPS files share a common sort order, you can sequentially merge the data files (i.e. first NBER observation to first CPS observation, second to second, etc). I recommend checking values of sex, age, and race before and after merging the data. If your analysis is spanning many years you will also want to consult the Docs corresponding to your samples to make sure the MSA Rank codes correspond to your MSAs of interest (as rankings likely changed over time).
Also note the warning in the METFIPS variable description. As the CPS samples were not intended to be representative at the MSA level, only large MSAs will produce reliable estimates.
I hope this helps